1. Field of the Invention
The present invention relates to an apparatus and method for improving the economics of flue gas recirculation. In particular, the present invention relates to an apparatus and method for the minimization of oxides of nitrogen ("NO.sub.x ") in the exhaust gas of various combustion processes.
2. Related Art
The minimization of oxides of nitrogen ("NO.sub.x ") in the exhaust gas of various combustion processes has been mandated by pollution control agencies throughout the country. Oilfield steam generators represent one of many industrial combustion processes where implementation of NO.sub.x control has become widespread. One of the ways in which NO.sub.x control is accomplished is to recirculate a percentage of the flue gas, or exhaust gas, (often 10%-20%) to mix with the inlet air.
The conventional NO.sub.x control systems, which utilize Flue Gas Recirculation ("FGR"), are extremely costly to manufacture, operate, and maintain due to the elaborate control systems that are required for effective performance of the system. Additionally, such systems, while substantially increasing generator operating cost, typically reduce the effective generator capacity.
One method of controlling the flue gas recirculation percentage is to provide control valves within the system to adjust the mixture of ambient air and flue gas to obtain a desired percentage. In the system disclosed in U.S. Pat. No. 5,040,470, the percentage of recirculated flue gas in the air inlet stream must be controlled to a reasonable precision. If this percentage is too high, generator output is reduced. If the percentage is too low, NO.sub.x reduction is insufficient. In this system, a control valve in the recirculation line is used to control the amount of flue gas recirculation. This valve works in tandem with the main air inlet valve, which controls the ambient air inlet rate. The main air inlet valve disclosed in U.S. Pat. No. 5,040,070 is itself a replacement for adjustable dampers, which are typically located between the blower and the burner section on units without NO.sub.x control. Under typical operating conditions, the main air inlet valve will be a source of significant frictional pressure drop, which must be overcome with additional energy consumption at the blower. To monitor the recirculation percentage, oxygen sensors are utilized. These sensors are used to monitor the percentage of flue gas recirculation, providing feedback to a recirculation rate control valve. The oxygen sensors add significantly to the expense of the system.
Conventional systems use active control in order to reach and maintain a selected recirculation percentage. Active control involves constant adjustment of control valves, and heat exchangers in order to operate the system at the selected recirculation percentage. Operators manually change valve settings or damper positions. In other systems, sensors integrated with control systems cause the system to alter relative valve positions and thermal conditions of the system in order to maintain the selected recirculation percentage. This active control adds significantly to the installation and operational costs of NO.sub.x control systems.
Other patents disclose controlling flue gas recirculation through an elaborate array of specialized burners, fans and dampers that alter the temperature of the flue gas in order to reduce noxious emissions. Such a system is disclosed in U.S. Pat. No. 4,659,305. This system uses conventional dampers and air diffusers, in conjunction with a recirculation fan, to enable the flue gas to be recirculated.
It is also desirable to calculate the flue gas recirculation percentage for monitoring purposes. Several conventional methods exist to calculate the percentage of flue gas recirculation (FGR). For example, oxygen sensors have been used to monitor the amount of oxygen in the combustion air. The percentage of FGR is then calculated from the oxygen sensor readings. However, the oxygen sensors are costly to install and maintain. Monitoring the flue gas recirculation percentage has also been achieved by metering the flow rate of the flue gas returned, or by performing a material balance using the temperature at different points in the system. However, the equations utilized to perform these calculations typically amount to a rough approximation of the actual flue gas recirculation percentage. The equations are generally inaccurate because they do not account for the heat added to the system by the mechanical inefficiencies in the blower.
Thus, there is a need in the art for a simple and inexpensive system for controlling flue gas recirculation percentage to minimize NO.sub.x emissions. Particularly, there is a need in the art for a system that eliminates the need for active control to maintain a selected flue gas recirculation percentage. There is a further need in the art for an improved method for calculating flue gas recirculation (FGR) percentage for monitoring purposes.